Delineation of the nature of the surface of biological apatites is a prerequisite to understanding the chemical behavior of these minerals in tooth, bone and other calcified tissues; variation in surface composition not only influences the composition of the adjacent equilibrium solution but also affects the extent of adsorption of ions and molecules from that medium. The objective of the proposed program is to determine, by means of a radioisotope dilution technique, the standard state surface concentrate of calcium and phosphorus on hydroxyapatite, fluorapatite and carbonate- and sodium-substituted apatities in aqueous media spanning the physiological pH, calcium and phosphorus range. Following the preparation and characterization of the apatites, samples of each will be successively pre-equilibrated in aqueous solutions of constant ionic strength at a given pH until their surface composition becomes invariant. The equilibrated suspensions will be dosed with Ca45 and P32 and, after the attainment of isotopic equilibrium, the solution concentrations of these isotopes and of total calcium and phosphorus will be determined. Assuming constant specific activity of isotopes between the surface of the apatites and the equilibrium solution, the number of calcium and phosphorus atoms per unit area of the surface can be calculated at any pH given the specific surface of the solids.